Blow-by gas is leakage of an air-fuel mixture or combustion gas from a combustion chamber of an internal combustion engine into a crankcase through a gap between a piston and a cylinder (specifically, through a gap between a piston ring and a cylinder). That is, blow-by gas contains unburned gas, exhaust gas, and what is called oil mist, which is a mixture of the foregoing and engine oil (hereinafter simply referred to as the oil). The entry of the blow-by gas into the crankcase causes deterioration of the engine oil, corrosion of metal, and contamination of the atmosphere.
Addressing thereto, what is generally practiced is provision of a blow-by gas return apparatus, that is, a mechanism that returns blow-by gas accumulated in the crankcase to an intake passage, so that the returned blow-by gas is mixed with a new air-fuel mixture and burned and thereby prevented from being released into the atmosphere as it is. However, since blow-by gas contains not only oil mist but also moisture contained in exhaust gas, in some cases, returning the blow-by gas into the intake passage as it is works adversely.
In view of the foregoing, a blow-by gas return apparatus is provided with an oil separator for trapping and removing mainly an oil component in blow-by gas, in order to remove liquid components such as oil (oil mist) and water contained in blow-by gas as much as possible and return the blow-by gas to the intake passage. There exists known engine that is externally equipped with an oil separator as an independent component, which is disclosed in Patent Documents 1 and 2. Patent Document 1 discloses the oil separator as a ventilator (2), and Patent Document 2 discloses the oil separator as a ventilator apparatus (1).
The blow-by gas return apparatus including a pipe for returning blow-by gas to the intake passage is basically externally mounted on an engine and exposed outside and, therefore, tends to be susceptible to cold. That is, under an extremely low temperature condition such as −20° C. to −30° C. in northern countries in winter, cooled blow-by gas makes moisture in the blow-by gas freeze, possibly resulting in clogging.
Particularly, an oil separator externally mounted on an engine has a large surface area and tends to be cooled, inviting freezing of moisture in the blow-by gas contained therein. The frozen moisture not only hinders the blow-by gas returning function, but also may clog the retuning port for trapped oil, inviting an excessive accumulation of the oil inside the oil separator, whereby the oil separation function is hindered. Also, the clogging may increase the internal pressure in the crankcase, which may result in unexpected oil leakage.
Addressing the problems, as disclosed in FIG. 1 of Patent Document 2, there is known a technique of providing an antifreeze cover (26) including an insulator member (28) covering the outside of the bottom wall of an oil separator, thereby preventing the inside of the oil separator from being excessively cooled.